As the United States and its allies weigh limited military strikes against Syria, their lawyers have been exploring a range of legal frameworks for any operation, including propositions that members of the international community have the right to use force to protect civilians or to deter a rogue nation from using chemical weapons.

But the Obama administration’s efforts to build a legal case are encountering skepticism from U.N. officials and other experts, including former Republican and Democratic State Department lawyers, who argue that the use of force against the Syrian regime, absent a U.N. Security Council resolution, would be illegal.

“Using force in a situation like this could be seen as legitimate internationally and the right thing to do; that’s the policy­makers’ call,” said David Kaye, a former State Department lawyer who teaches international law at the University of California at Irvine. “But that’s different from saying it would be legal. It wouldn’t be, unless you had authorization of the Security Council.”

Kaye and other legal scholars say the U.N. Charter explicitly prohibits the use of force against other U.N. members, except in self-defense against an imminent threat or in an operation authorized by the 15-nation Security Council.

Although Britain said Wednesday that it would seek a Security Council resolution authorizing the use of force against Syria, the prospects for approval appear dim, given firm opposition from veto-wielding members Russia and China.

“I think that international law is clear on this,” said Lakhdar Brahimi, the U.N.-Arab League special envoy for Syria. “International law says that military action must be taken after a decision by the Security Council. That is what international law says. What will happen, then again, I don’t know.”

Speaking late Tuesday at the Peace Palace in The Hague, U.N. Secretary General Ban Ki-moon said the use of chemical weapons “would be an atrocious violation of international law.” Without singling out the United States directly, he counseled states to address the crisis in Syria through diplomatic means. “Here in the Peace Palace, let us say: Give peace a chance,” Ban said. “In this hall dedicated to the rule of law, I say: Let us adhere to the United Nations Charter.”

He added: “The military logic has given us a country on the verge of total destruction, a region in chaos and a global threat. Why add more fuel to the fire?”

A senior Obama administration official said the United States is exploring a number of possible legal arguments to justify an armed response to what officials believe is the worst chemical weapons attack since 1988, when then-Iraqi leader Saddam Hussein gassed more than 3,000 Kurds in the town of Halabja. The official spoke on the condition of anonymity to discuss internal deliberations.

As part of that effort, U.S. officials are examining international agreements, including the 1925 Geneva Protocol and the 1992 Chemical Weapons Convention, both of which ban the use of chemical arms. “The fact that there is a long-standing international norm around the use of chemical weapons, that provides legitimacy for the international community to respond,” the administration official said.

Although it would be a stretch to argue that Syria’s use of chemical weapons, if proven, constitutes a threat to the United States, the administration is also studying the possibility that U.S. force could be used in support of Syria’s neighbors, including American allies Jordan and Turkey, if those governments invoke the right to self-defense against Syria.

French President François Hollande invoked the responsibility-to-protect doctrine in making his case to “punish those who took the decision to harm the innocent.” In a speech to French ambassadors Tuesday, Hollande suggested that international law has not kept pace with the emerging international consensus on the need to act to halt mass atrocities. “International law must evolve with the times. It cannot serve as an excuse to allow mass murder,” he said.

U.S. presidents from both parties have long held that they are free from international constraints to use force in defense of American interests.

The United States and its allies have frequently undertaken military actions without Security Council approval. In 1999, the Clinton administration led a NATO air war in Kosovo. About four years later, the George W. Bush administration invaded Iraq without an explicit authorization from the U.N. Security Council.

The Obama administration also has asserted the right to act without the council’s endorsement. “When U.S. national security is threatened and the Security Council is unwilling to authorize the use of force but the president believes it is judicious to do so, of course that is something that he should be free to do,” Samantha Power, the U.S. ambassador to the United Nations, said last month at her confirmation hearing.

But U.S. government lawyers have been less willing to claim a special legal right to act. In Kosovo, Clinton-era policymakers were never able to prevail upon the State Department’s lawyers to characterize the NATO action as legal. Instead, the lawyers agreed to call it legitimate. In Iraq, the Bush administration argued that its legal authority derived from a 12-year-old resolution that ended the Persian Gulf War, a contention that was hotly contested by then-U.N. Secretary General Kofi Annan, who termed the Iraq war illegal.

In recent days, senior Obama administration officials have privately invoked the Kosovo intervention as a possible precedent. But legal scholars say that could prove problematic, because the United States has never released a legal defense of its decision to attack Serbia to halt mass abuses against ethnic Albanians in Kosovo. In authorizing the use of force, the Clinton administration argued that NATO’s military intervention in Kosovo should not constitute a precedent.

“It doesn’t provide a legal basis” for acting, said John Bellinger III, who served as lawyer for the State Department and the National Security Council during Bush’s second term. “Lawyers in both Republican and Democratic administrations have been concerned that a doctrine of humanitarian intervention could set a bad precedent that would be more likely to be used by other countries like Russia and China or some African countries,” which could invoke the principle of “humanitarian intervention” in attacking their enemies.

Both the Kosovo operation and the 2011 air assault in Libya, in which the United States participated, were launched with consensus within NATO. The alliance’s ambassadors have met to discuss the Syria situation. But the possibility of NATO intervention, which must begin with a request from a member nation under threat, has not yet been raised.

DeYoung reported from Washington.